You are supposed to inform DVLA of any change to fuel type when it is converted but not everyone does (2 out of my 3 LPG cars had petrol as the fuel type on the V5 when I got them) and you are supposed to declare it as a modification when you insure it. In both these cases they are likely to want to see some sort of documentation to show that it has been done and is safe. They may accept a document from a company not in the UK or you may need to take it to an installer here to get it checked and a safety certificate issued.

The gas installation isn't checked as part of the MoT but if you present the car for test running on gas, it will be tested to a lower standard non-cat test than if you take it there running on petrol. They won't worry about where it was converted as long as there isn't anything about the gas system that looks obviously dodgy or dangerous.